Universe is Expanding

The primary message of this article is that the Universe is expanding,
as shown by a linear relationship between the distance of a galaxy and
the velocity at which is receding discovered by Edwin Hubble. A
secondary message is that the cosmological constant, which Einstein
added to his equations of General Relativity to account for a static
Universe, may not be necessary, showing that even Einstein can be wrong
occasionally.

This article introduces the idea of an expanding Universe. When we
left the Cosmic Times in 1919, Albert Einstein had added the
Cosmological Constant to his equations of General Relativity to prevent
the Universe from expanding. The idea of an expanding Universe was
against his philosophical viewpoint, and nothing was violated by adding
this Cosmological Constant to his equations.

Redshift

One key concept to understanding Hubble's results is understanding
the idea of redshift. Often this idea is introduced alongside the
Doppler effect for sound because students are much more familiar with
this  it is what causes the pitch of a siren to appear to drop as
it passes by a listener. For light, when a source is emitting light
while it is moving away from us, the wavelength of the light as we see
it will be shifted toward the red, or toward longer wavelengths.

When astronomers look at light from distant galaxies, they can
identify signatures of certain elements in the form of "spectral lines".
By looking at how much these signatures are offset from their "at rest"
wavelengths, astronomers can tell how fast the object that emitted them
was moving. If the object is moving away from the observer, the light
is shifted toward the red, or toward longer wavelengths, and this is
often called "redshift". If the object is moving toward the observer,
the light is shifted toward the blue, which is often called
"blueshift".

In 1917, an astronomer named Vesto Slipher, discovered that the light
from several "nebulae" (later found to be galaxies) was "redshifted".
It is commonly thought that Hubble discovered the redshift of galaxies,
but this is not true. Hubble found the relationship between redshift
and distance that is discussed in this Cosmic Times article, but he was
not the first to see the redshift itself.

Expanding Universe!

When Hubble plotted up the distance of the galaxies in his sample
versus the redshift of those galaxies, he found that there was a trend
 the further a galaxy was, the faster it was moving away. This is
exactly what would be expected if the Universe was expanding.

This is one concept that your students might struggle with  why
does an expanding Universe imply that the further a galaxy is from us,
the fast it would be moving away from us. To help illustrate this,
imagine that this picture shows several galaxies in the Universe as they
were in the past:

Let's say that our galaxy is the blue star near the middle of the
picture. Then let the Universe expand for a while and look at what's
happened:

We are the blue galaxy, so we haven't moved. However, all of the
other galaxies appear to have moved away from us. And, if you
examine the size of the arrows from the original galaxy positions to the
new ones, it is clear that the further a galaxy was away from us, the
faster it appears to have moved, in the same amount of time.
That means that the further away the galaxy was, the faster it appears
to move away from us as the Universe expands.

One misconception that your students may have is that because all of
the galaxies appear to be moving away from us, that means that we must
be the center of the Universe. This is not true. Any galaxy would
actually perceive the same thing. Let's say that instead of being the
blue galaxy in that image above that we are the yellow galaxy. The
picture we would see after the expansion would then be this:

The yellow galaxy also sees all of the other galaxies moving away
from it, and the further the galaxy started from the yellow galaxy, the
more it moved.

The fact that we see the galaxies all moving away from us is
indicative of the expanding Universe, but not that we are the
center of the Universe. In fact, the expansion happens
everywhere.

Einstein still didn't believe

If you read the 1919 edition of the Cosmic Times, you know that
Einstein did not like the idea of an expanding Universe. In fact, even
though his equations of General Relatively suggested an expanding
Universe, Einstein added his "Cosmological Constant" to those equations
to prevent expansion.

As of the publication date of this edition of the Cosmic Times,
Einstein is still not convinced of Hubble's results. However, in 1930,
Einstein travels to Mount Wilson to talk with Hubble and see his work
first-hand. After meeting with Hubble, Einstein does become convinced
that the Universe is, indeed, expanding.

Hubble's work holds up

Hubble's results have proved to hold over much greater distances than
Hubble himself could probe. As we've developed larger and more
sensitive telescopes and as we've determined more ways to pinpoint
distances in further and further objects, we have continued to find that
the further a galaxy is from us, the faster it is moving away, as this
image shows for just a sample of the currently available data:

The relationship between a galaxy's
distance and its speed holds far beyond what Hubble originally
measured. This diagram uses modern measurements to show the same
relationship. The gray box shows the region that Hubble probed.
Data from the Hubble Space Telescope Key project, courtesy Prof.
John Huchra.

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